r - How can I extract the month using sqldf package -


i tried view based on group of date using sqldf package , month function got error :error in sqlitesendquery(con, statement, bind.data) : error in statement: no such function: month

here query: s<-sqldf("select month(datetime),sum(wolfs) group datetime")

attached toy data frame:

 df <- read.table(text = "datetime         birds    wolfs     snakes                          2014-05-21        9         7                             2014-04-28        8         4    b                          2014-04-13        2         8    c                          2014-03-12        2         3                             2014-02-04        8         3                             2014-02-29        1         2                             2014-01-17        7         1    b                          2014-01-16        1         5    c                          2014-09-20        9         7    c                          2014-08-21        8         7    c ",header = true)

how can extract month using sqldf package?

i suspect used sql server, sqldf backend being used in case sqlite, there no month function. try instead: 

r> sqldf("select strftime('%m', datetime) month             ,sum(wolfs) wolves          df          group strftime('%m', datetime)") #    month wolves #  1    01      6 #  2    02      5 #  3    03      3 #  4    04     12 #  5    05      7 #  6    08      7 #  7    09      7

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